Austenitic Stainless Steel

ABSTRACT

A new austenitic stainless steel with the following composition by weight is described: 0.03%&lt;carbon&lt;0.07%, 7.0%&lt;manganese&lt;8.5%, 0.3%&lt;silicon 0.7%, sulphur&lt;0.030%, phosphorus&lt;0.045%, 16.5%&lt;chromium&lt;18.0%, 3.5%&lt;nickel&lt;4.5%, 0.1%&lt;molybdenum&lt;0.5%, 1.0%&lt;copper&lt;3.0%, 0.1%&lt;nitrogen&lt;0.3%, the difference consisting in iron and common process impurities. The steel thus obtained has an optimum combination of corrosion resistance, deformability and work-hardening properties, which make it suitable as a substitute for normal steel type  1.4301  in various specific applications.

The present invention relates to a new austenitic stainless steel with alow nickel content which has special characteristics in terms ofcorrosion resistance in given environments, deformability andsuitability for work-hardening. The steel according to the presentinvention is characterized by the following chemical composition:

-   -   0.03%<carbon<0.07%    -   7.0%<manganese<8.5%    -   0.3%<silicon<0.7%    -   sulphur≦0.030%    -   phosphorus≦0.045%    -   16.5%<chromium<18.0%    -   3.5%<nickel<4.5%    -   0.1%<molybdenum<0.5%    -   1.0%<copper<3.0%    -   0.1%<nitrogen<0.3%        the difference consisting in iron and common process impurities.

A very important characteristic of the new steel is the small amount ofnickel it contains: it is in fact well known that the price of thiselement is unstable, with a continuous tendency to increase, resultingin continuous variations in the costs of the articles produced withmaterials which contain this element.

BACKGROUND ART

Austenitic stainless steel is an iron and carbon alloy containingvarious other elements, the main ones of which are chromium and nickel.The combination of these elements gives the steel a basic property ofcorrosion resistance owing to the formation of a protective surface filmwhich is due to the presence of a chromium content of at least 1.11% andwhose qualities are improved by the presence of nickel and otherelements. Other typical properties of austenitic stainless steels arethe very low magnetic permeability (non-magnetic property), heatresistance, cold deformability and suitability for work-hardening. Owingto these properties, austenitic stainless steels are used in a very widerange of applications.

1.4301 Steel

The most well known and widely used type of austenitic stainless steelcontains about 18% chromium 10% nickel and has always been referred toas 18/10 steel. In the European standard EN 10088-3 1997 this steel hasbeen called X5CrNi18-10 and has been attributed the steel number 1.4301.In the United States standard AISI this steel is called 304. Thepercentage by weight chemical composition envisaged for this steel bythe European standard is as follows:

-   -   C=0.07 max    -   Si=1.00 max    -   Mn=2.00 max    -   P=0.045 max    -   S=0.030 max    -   N=0.11 max    -   Cr=between 17.00 and 19.50    -   Ni=between 8.00 and 10.50

In the case of products which are intended to be machined, the samestandard envisages a variant whose sulphur content is controlled (or“micro-resulphurised”) where

-   -   S=between 0.015 and 0.030

It should be noted that the maximum sulphur content coincides with thatof basic steel, so that in fact this is not another steel, but only avariation of the same type 1.4301 obtained by dividing the analyticalrange permitted by sulphur. Sulphur has the capacity to weaken themetallic matrix and therefore improve the machinability during the swarfremoval operations. At the same time, however, sulphur, even thoughpresent in limited amounts, modifies the corrosion resistance. Thismicro-resulphurised variant is cited here because below it will often beused for comparison with the type 1.4301 steel and with the steel ofthis invention.

1.4301 steel has extremely broad technological and corrosion propertiessuch it has been become very widely established in the engineeringsector as a structural material as well as in the environmental sector:it is in fact widely employed in the transportation, architecture andthe domestic sectors, being used at high temperatures and in corrosiveenvironments. The type 1.4301 is the most well known, widespread andresearched in the sector of austenitic stainless steels and therefore isused as a reference type for comparing the characteristics of otheraustenitic stainless steels.

Other Comparison Steels

There exist other steels with a similar composition which differ owingto small analytical variations of a certain element which give them animproved property. Some of these steels are mentioned here because belowthey have been used for comparison with the steel according to theinvention in order to highlight its characteristics. The type1.4307—X2CrNi18-9 (AISI 304L in the US standards) is a steel similar tothe preceding one, but with a limited carbon content which improves theintergranular corrosion resistance. The chemical composition of type1.4307 steel is as follows:

-   -   C=0.03 max    -   Si=1.00 max    -   Mn=2.00 max    -   P=0.045 max    -   S=0.030 max    -   N=0.11 max    -   Cr=between 17.50 and 19.50    -   Ni=between 8.00 and 10.00

The type 1.4306—X2CrNi19-11 is a further low-carbon variant with agreater content of nickel which is added in order to improve the colddeformability and the corrosion resistance. The chemical composition ofthis type is as follows:

-   -   C=0.03 max    -   Si=1.00 max    -   Mn=2.00 max    -   P=0.045 max    -   S=0.030 max    -   N=0.11 max    -   Cr=between 18.00 and 20.00    -   Ni=between 10.00 and 12.00

The type 1.4567—X3CrNiCu18-9-4 is a version with the addition of copperin large amounts for the purpose of improving the cold deformability: itis used for those particular cold-pressed products where the precedingtypes are unable to withstand the extreme deformation, such as, forexample, hexagonal socket head screws. The chemical composition is asfollows:

-   -   C=0.04 max    -   Si=1.00 max    -   Mn=2.00 max    -   P=0.045 max    -   S=0.030 max    -   N=0.11 max    -   Cr=between 17.00 and 19.00    -   Cu=between 3.00 and 4.00    -   Ni=between 10.00 and 12.00

The Characteristics of Austenitic Stainless Steels

The main characteristics of an austenitic stainless steel are itscorrosion resistance, non-magnetic nature, cold-deformability andsuitability for work-hardening. These characteristics are obtained bymodifying various factors, including the chemical composition: inaddition to chromium and nickel, the other secondary elements have animportant effect. The effect of chromium, referred to as “alphagenic”,tends to stabilize the ferritic phase of the materials (alpha phase):other elements, such as silicon and molybdenum, behave in the samemanner as chromium, although to a lesser degree. The same applies tonickel, which is a “gammagenic” element, and therefore has a stabilizingeffect on the austenitic phase (gamma phase): various elements such ascarbon, nitrogen, copper and manganese behave in the same manner asnickel.

The Nickel Content of Austenitic Stainless Steels

Most of the known austenitic stainless steels used on the market havenickel contents of about 8-10%, as in the case of the types mentionedhitherto. During the last few years, the worldwide economic situationhas resulted in the price of nickel being very unstable, with a markedtendency to increase.

Manufacturers and retailers of stainless steels therefore havedifficulty in operating within a fluctuating market, so much so thatnowadays in Europe the price of these products is composed of a baseprice and an additional price, referred to as “alloy add-on”, which isdefined at the time of delivery: the “alloy add-on” varies withpredefined mechanisms depending on the value of nickel on the worldmarket. Steel product processing companies, for their part, havedifficulty in establishing the prices of the parts produced since theycannot know the exact price of the raw material until the time ofdelivery.

For this reason, different austenitic stainless steels with low nickelcontents have been researched: some of these, which are more widely usedand have been known for some time, are included in various standards andused because of their specific characteristics. Others have beenrecently developed with the aim of obtaining some of the basiccharacteristics of austenitic stainless steel. In fact, by suitablyincreasing the content of the less costly “gammagenic” elements(nitrogen, copper and manganese), it is possible to obtain an austeniticstainless steel which is equally stable, but has a low percentagecontent of nickel (and therefore a price which is less dependent on thefluctuations of the cost of nickel) and with one or more technologicalproperties the same as those of normal conventional austenitic steelswith a higher nickel content. Austenitic steels with a low nickelcontent are for example described in EP593158, EP694626, EP896072,EP969113 e WO 00/26428.

SUBJECT OF THE INVENTION

The subject of the present invention is a steel having a nickel contentwhich is markedly lower than that of basic steel type 1.4301 (AISI 304)and which, with suitable balancing of the other elements, has manyproperties similar to the corresponding properties of basic steel type1.4301 (AISI 304); it has the composition shown below:

-   -   0.03%<carbon<0.07%    -   7.0%<manganese<8.5%    -   0.3%<silicon<0.7%    -   sulphur≦0.030%    -   phosphorus≦0.045%    -   16.5%<chromium<18.0%    -   3.5%<nickel<4.5%    -   0.1%<molybdenum<0.5%    -   1.0%<copper<3.0%    -   0.1%<nitrogen<0.3%        where the difference consists in iron and common process        impurities.

The steel according to the present invention may be obtained by means ofthe conventional processes for the preparation of austenitic stainlesssteels, such as those for example described in “ASM SpecialtyHandbook—Stainless Steels” edited by “The Material InformationSociety”—USA. Preferably it has the composition indicated below:

-   -   0.04%<carbon<0.06%    -   7.5%≦manganese<8.0%    -   0.4%<silicon 0.6%    -   0.002%<sulphur 0.004%    -   0.030%<phosphorus<0.035%    -   17.0%<chromium<17.5%    -   3.8%<nickel<4.2%    -   0.1%<molybdenum<0.3%    -   2.0%<copper<2.5%    -   0.15%<nitrogen<0.2%

According to one of the possible embodiments of the invention, thesulphur is less than 0.005%. According to another possible embodiment,which does not exclude the previous embodiment, the nickel is higherthan 4.0%. According to the best embodiment of the invention, the carbonis about 0.055%, the manganese is about 7.50%, the silicon is about0.52%, the sulphur is about 0.003%, the phosphorus is about 0.032%, thechromium is about 17.0%, the nickel is about 4.0%, the molybdenum isabout 0.19%, the copper is about 2.0% and/or the nitrogen is about0.17%.

In order to define the characteristics of the product obtained with thenewly invented steel, its main performance features have been studiedand compared with those normally encountered in basic 1.4301 steel andsimilar steels: the results have proved to be very positive since, forthe same functional characteristics, the cost of the steel is decidedlylower than that of basic steel type 1.4301 and in any case not soclosely dependent on the nickel market.

The characteristics considered on the pages below have been obtained bymeans of varying castings of the new steel, all carried out withanalyses similar to that of the best embodiment mentioned above.

Stress Corrosion Cracking

The steel according to the present invention presents a higherresistance to “stress corrosion cracking” (also called “delayedcorrosion”) than the steels commonly known in the art and, inparticular, than those disclosed by WO 00/26428, EP896072 or EP969113.Such a higher resistance can be explained through the selected nickelrange of between 3.5 and 4.5% by weight, as for instance subsequentlydemonstrated by J. Charles, Stainless Steel '05, Proceedings of the5^(th) European Congress Stainless Steel Science and Market, Seville,Sep. 27-30, 2005 (pages 19-26).

This improved resistance to “stress corrosion cracking” makes the steelof the present invention particularly suitable for the manufacture ofwires having a “deep drawing ratio” and which could be exposed toaggressive environments as for instance wires for agricultural use,electric household appliances, bicycle spokes; wires for laundry dryingframes; wires for architecture, for meshwork and for hooks used on slateroofs.

Cold Deformability by Means of Drawing

For the reduction in cross-section r the following relation isapplicable:

$r = {\frac{A_{0} - A_{1}}{A_{0}} \cdot 100}$

where:

-   -   A₀=Initial cross-sectional area    -   A₁=Final cross-sectional area

The drawing of the rolls is performed by means of successive passesthrough the tools (drawing dies) which deform the product, graduallydecreasing its cross-section.

During deformation, a phenomenon called work-hardening, proportional tothe reduction, occurs, said phenomenon resulting in an increase in thetensile properties of the material (R_(m), R_(p(0,) ₂₎) and a decreasein the plastic properties (A, Z), up to the point where the material isno longer deformable. When work-hardening is such that the material nolonger possesses plasticity, the wire breaks during further passesthrough the drawing dies and the product can no longer be drawn.

Under normal conditions with multiple-pass drawing machines operating atsuitable industrial speeds, the reference stainless steel 1.4301 (AISI304) is able to withstand drawing reductions of up to 88%. Beyond thesevalues the work-hardening is such that the material breaks and is nolonger capable of being deformed.

The stainless steel according to this invention, under identicalconditions, is able to withstand drawing with reductions in thecross-section in the region of 92-94%.

This data is very important for detailed work where small diameters ofthe drawn wire are required, with the result that a certain amount ofannealing during the reduction cycles may be dispensed with.

Table 1 shows the tensile strength and elongation at break values of thesteel according to the invention for various degrees of reduction duringdrawing, compared with two reference steels: steel type 1.4307 with alow carbon content (about 0.02%) and steel type 1.4301 with a slightlyhigher carbon content (0.04%).

TABLE 1 Mechanical properties depending on work-hardening New steel1.4307 low C 1.4301 R A R A R A % reduction MPa % Mpa % MPa % 0 659 42580 42 569 42 17.4 770 23 810 35.1 1045 12 952 12 56.4 1390 3.5 1140 4.067.9 1420 4.0 70 1583 2.5 1320 3.0 76 1610 1.5 84 1803 1.5 1490 2.5 17001.5 87.7 1750 1.5 90.3 1932 1.2 92 2000 1.0

FIG. 1 shows in graph form the tensile strength values as a function ofthe drawing reduction for these steels, while FIG. 2 shows the same typeof comparative graph relating this time to the percentage elongation atbreak value.

The work-hardening is due to the partial and progressive transformationof part of the austenite into martensite, which is the hardest componentof steel. A metallographic study was carried out on samples taken frommaterials in the annealed and work-hardened state, these revealing boththe deformation of the grain, with elongation in the drawing direction,and the austenite-martensite transformation.

FIG. 3 shows a longitudinal metallographic cross-section through theproduct in an ultra work-hardened state of the wire obtained with thenew steel, in which the work-hardening lines due to the martensitictransformation are clearly visible.

FIG. 4 shows the same type of cross-section carried out on a sample ofthe reference steel type 1.4301 (AISI 304).

Relative Magnetic Permeability μ_(r)

The relative magnetic permeability measures the ratio between themagnetic permeability of a material μ and that of a vacuum μ₀.

$\mu_{r} = \frac{\mu}{\mu_{0}}$

The magnetic permeability of a material μ (measured in Henry/metre[H/m]) is defined by the ratio between the magnetic induction value Band the value of the magnetizing force H.

The magnetic permeability of the vacuum is equal to μ₀=1.256×10⁻⁶ H/m.

The magnetic permeability of a material basically measures theferromagnetism, i.e. the property of a steel to react with a magneticfield of given value.

In the case of stainless steels, the martensitic structure isferromagnetic (μ_(r)=700-1000), while austenite is practicallynon-magnetic (μ_(r)<1,2).

An austenitic steel in the solubilized state, and hence with a totallyaustenitic structure, is completely non-magnetic: when it is subjectedto a magnetic field, for example that of a magnet, it does not react.

An austenitic steel in the work-hardened state, for example afterundergoing drawing reductions, is increasingly more magnetic dependingon the percentage of austenite transformed into martensite (basicallydependent on the drawing reduction and the chemical composition).

For this reason, a steel type 1.4301 (AISI 304) which in the solubilizedstate is non-magnetic, after reductions with value of about 65%, has astructure which is partially ferromagnetic with a relative magneticpermeability of about μ_(r)=1.50 (with a magnetic field of 4000 A/m);after reductions of 85%, its relative magnetic permeability rises toμ_(r)=2.20 with the same magnetic field.

The steel according to the present invention remained perfectlynon-magnetic also following numerous drawing operations: under the sametest conditions, with reductions of 65%, we obtained a permeabilityμ_(r)=1.10, while with reductions of 85% the permeability rose only toμ_(r)=1.30.

The magnetic permeability in a stainless steel assumes particularimportance both in the case of more complex applications (e.g. solenoidvalve bodies, where the part must not be influenced by the magneticfield of excitation of the valve), but also for more straightforwardapplications, where recognition of the material is simply carried out bymeans of a magnet, as in the case of laundry drying frames sold atmarkets or in supermarkets: if the wire of the laundry drying rack isnot attracted by the magnet, it is recognised as being austeniticstainless steel and is much more highly valued than the correspondingwire made of ferritic stainless steel or even galvanized iron, which areboth highly ferromagnetic. The possibility of obtaining drawn wires withhigh work-hardening values (required by the product itself in order towithstand the load of wet laundry), without any significant variation inthe magnetic permeability, results in the invention being particularlysuitable for this type of use.

Cold Deformability by Means of Pressing

Tests for the production of screws by means of cold deformation werecarried out as follows:

-   -   Hexagonal-head screws (DIN 933 M5×25): for this product a steel        type 304L with Cu content of about 0.9% is used.    -   Socket-head cap screws (DIN 912 M5×12): for this type of product        normally a steel type 304Cu is used, with the addition of 3-4%        Cu in order to improve the deformability.

The characteristics of the screws produced were determined by means oftensile tests carried out in accordance with the standard UNI EN ISO3506 part 1 edition February 2000 and HV 500 microhardness tests.

The results of the tensile test are shown in Table 2

TABLE 2 Results of tensile tests carried out on cold-pressed screwsUpper Breaking yield Elongation load point at break Type of R_(m)R_(p(0, 2)) A screw Dimensions Material Mpa Mpa % DIN 933 M 5 × 251.4306 967 754 2.7 Sheared (304L) hexagonal New steel 1137 887 2.8 headscrew DIN 912 M 5 × 12 1.4567 865 675 2.3 Socket (304Cu) head New steel1160 905 2.2 screw

FIG. 6 shows the microhardness values determined at various points inthe longitudinal section of the screws DIN 933 M5×25 produced.

In the same manner, FIG. 7 shows the microhardness values detected atvarious points of the cross-section of screws DIN 912 M5×12.

Before commenting on these results, it should be noted that thereference standard for stainless steel screws (UNI EN ISO 3506-1“Mechanical properties of corrosion-resistant stainless steel connectingelements—screws and stud screws”) does not permit at the moment thistype of austenitic steel. It may, however, be possible to apply for andobtain inclusion of the newly invented type in the future standard forscrews, thus allowing its use.

The comparisons have been made, as always, with screws made of normalsteel type 1.4301 (AISI 304). The screws made with the steel accordingto the present study had a higher tensile strength of about 70 MPa inthe case of hexagonal head screws and 95 MPa in the case of socket headscrews: this greater difference is due to the very poor work-hardeningproperty of the 304Cu steel used for the comparison. Likewise, thehardness values are about 100 HV points higher in the case of the steelaccording to the invention. All the mechanical properties recorded are,however, within the limits stipulated by the standard for quality A4screws (corresponding to the reference steel 1.4301) with strength class70 or 80 (relating, therefore, to “work-hardened” or “ultrawork-hardened” materials).

These results of pressability must be related to the technicalpossibility of producing screws by means of cold deformation using thenew steel. Considering the corrosion-resistance properties of this steel(described in the following paragraphs), it seems possible to request,in due course, broadening of the range of steels accepted for theproduction of screws, at least as regards the strength class 80 (that ofultra work-hardened steels), which is sometimes difficult to achievewith normal austenitic steels.

Corrosion Resistance of the Semifinished Starting Product

Corrosion-resistance tests were carried out using samples obtained bymeans of machine-tool processing of solubilized wire rod.

The types of steel which underwent the tests were, in addition to thesteel of the present invention, also two castings of austenitic steeltype 1.4307 consisting of the micro-resulphurised variant (S=0.030 formachine-tool processing) and the variant with a very low sulphur content(0.003).

The tests carried out and the corresponding reference standard, whereapplicable, are listed in Table 3.

TABLE 3 Corrosion tests carried out on samples obtained from solubilizedwire rod Test in 20% — 1 cycle of 96 hours sulphuric acid at +20° C.Test in 65% nitric ASTM A262 test 3 cycles at 48 hours acid C at boilingtemperature - change of solution with each new cycle Test in 6% ferricASTM G-48 1 cycle of 72 hours chloride at 22° C. +/− 2

The results for the test with 20% sulphuric acid are shown in the graphsof FIG. 9. Similarly, FIG. 10 shows the results of the test in 65%nitric acid carried out on the same steels. In FIG. 11, the corrosiontest was carried out in 6% ferric chloride.

From the results it can be easily understood that in this type of test,the progression of the corrosion is greatly influenced by the sulphurcontent of the steel, while the decidedly lower nickel content did notresult in a substantial deterioration.

The new steel in fact has a performance perfectly in keeping with thatof the reference types and only in the nitric acid test is the corrosionvalue slightly higher than that of the type 1.4307 micro-resulphurisedsteel.

Before reaching conclusions in connection with these tests it isnecessary to point out again that both the steels used for comparisonhad an extremely low carbon content (type 1.4307 corresponds to the typeAISI 304L, Low Carbon) : the new steel is therefore not affected, allother conditions being equal, by the C content which is higher than inthe basic comparison steels.

By way of conclusion, these tests show that the sulphur content in asteel type 1.4307 with a corrosion-resistance considerably higher thanthe basic type 1.4301) has a decisive influence on the corrosionresistance. Both the compared types (1.4307 steel with low sulphurcontent and micro-resulphurised steel) are able to form part of aperfectly compliant supply of “normal” 1.4301 steel since this typeenvisages only a maximum limit for the elements C (0.07 max) and S(0.030 max).

The steel according to the invention, in the solubilized state and ontest pieces obtained by means of machining, has corrosion-resistanceproperties which are practically the same as those of the referencesteels.

Corrosion Resistance of Drawn and Solution Annealed Steels

Corrosion tests were carried out, in different work-hardeningconditions, on some samples of drawn wire and drawn+solution annealedwire made from the new steel and, by way of comparison, various otherqualities of stainless steel.

Most of the tests were carried out in accordance with internationalstandards which describe the methods to be applied, but do not describethe threshold values (exposure time or the like) which must besurpassed: these threshold values are established contractually in eachcase during placing of the order. In the present test program onlycomparative tests were carried out between the new steel and somereference steels, subjecting all the parts together to variable exposuretimes, until oxidation appeared in some of the parts or for time periodswhich were sufficiently long to guarantee the applicability thereof.

Table 4 lists the types of materials which underwent this type of test,their diameters and the associated working conditions.

TABLE 4 Wire samples subjected to corrosion tests Quality European AISIDiameter Reference standard standard mm State number 1.4301 304 2.30Partially work- 1 hardened 1.4301 304 2.00 Solution annealed 2 1.4301304 1.30 Work-hardened 3 New steel 1.40 Solution annealed 4 New steel1.40 Work-hardened 5 New steel 2.25 Solution annealed 6 New steel 2.00Partially work- 8 hardened

Table 5 instead lists the tests which these samples underwent and thereference standards.

TABLE 5 Corrosion tests on wire Reference Test standard Duration Neutralsaline UNI ISO 9227 168/400 hours mist NSS Copper acetic acid UNI ISO9227 120 hours mist CASS Kesternich cycles DIN 50018 21 4 cycles of 24hours (corrosion in an consisting of 8 industrial hours exposure to SO₂atmosphere) and 16 hours exposure to the laboratory air Immersion testin — 168 hours a solution of NaCl 2M with pH 6.6 Intercrystalline ASTMA262 24 hours in corrosion test test E copper/copper sulphate/sulphuricacid solution

Outcome of Tests:

-   -   Neutral saline mist test

After exposure for 150 hours, no sample showed signs of corrosion.

Only after 200 hours were some spots of rust detected on the surface ofsamples 5 and 6 and some more extensive areas found on ferritic sample7.

After 400 hours these rust spots were extensive, so much so that theferritic steel was widely oxidised, while some rust areas affected thenew steel (the extent of these areas is proportional to the degree ofwork-hardening); at the same time only small sporadic spots appeared onthe type 1.4301 steel in the work-hardened state.

-   -   Copper acetic acid mist test

After 120 hours exposure, the behaviour of the various wires wassufficiently varied and it was possible to detect that the ferriticsteel 1.4016 had the most area covered by corrosion products (about40%).

The behaviour of the new steel and the 1.4301 steel is instead greatlyinfluenced by the degree of work-hardening: as known from theliterature, the best corrosion resistance is obtained with the materialin the solution annealed state, while it is worsened by work-hardening.It was noted, however, that the behaviour of the steel considered inthis study is midway between the type 1.4301 and the type 14016.

-   -   Corrosion tests in an industrial atmosphere using Kesternich        cycles

After 4 cycles the behaviour of the new steel was entirely similar toall the other types of austenitic steel, there being no appreciablecorrosion (the surface remained substantially unchanged).

-   -   Tests with immersion in a solution of NaCl 2M with pH 6.6:

In this case the best behaviour was that of the type 1.4301, followedvery closely by the new type, while the type 1.4016 had various rustspots.

-   -   Intercrystalline corrosion tests

After attack, all the test pieces were able to be bent through 180°without any signs of cracking or flaking on the surface subject totensile stress.

The corrosion tests carried out were particularly numerous and coveredall the possible ranges of applications such that it was possible todetermine the characteristics of the new material with a wide series oftests.

The tests were carried out on products in the wire state, in variousfinishing conditions, and confirmed, as is well known in the literature,that materials in the work-hardened state behave in general less wellwhen subjected to aggressive agents: the explanation of this phenomenonis due mainly to the tensioning of the grains and the grain edges whichmake the individual points more unstable and therefore more prone toattack and also the partial martensitic transformation, since thisstructure has a corrosion resistance which is less than that ofaustenite.

Overall the corrosion behaviour of the new steel was scarcely inferiorto that of the reference type 1.4301, for the same work-hardeningconditions.

Particularly positive was the behaviour in relation to atmosphericcorrosion and intergranular corrosion, where no differences were notedcompared to the reference type.

Additional tests in acid environment (H₂SO₄ 0.2 M+NaCl 1 g/l) evidencedthat the steel of the present invention also presents better anodicpolarization curves than similar steels having a lower nickel content.

Hot Tensile Strength

One of the main characteristics of stainless steels is the possibilityof use at high temperatures. Rapid hot tensile tests were carried out inorder to verify the mechanical properties at temperatures higher thanroom temperature. The samples of the new steel which underwent thistest, in the form of 3 mm diameter solubilized wires, were compared withidentical samples of 1.4307, 1.4310 and 1.4301 steel.

The tests were carried out at 900° C. in accordance with the standard EN10002 part 5, giving the results listed in Table 6.

TABLE 6 Mechanical properties during high temperature tests Test CrossTest Material piece sectional temper- European AISI diameter area atureR_(p0, 2) R_(m) standard Standard mm mm² ° C. MPa Mpa 1.4301 304 3 7.1900 91 141 1.4307  304L 3 7.1 900 94 155 1.4310 302 3 7.1 900 103 169New steel 3 7.1 900 90 155

The rapid hot tensile tests were carried out at a decidedly hightemperature (900° C.) compared to the operating temperatures normallypermitted. The results show that the new steel has a behaviour verysimilar to that of the normal reference steel, type 1.4301, while onlythe type with a higher carbon content (1.4310) has a slightly higher hotstrength, even though it as of the same order of magnitude.

High Temperature Stay Test

The basic stainless steel 1.4301 (AISI 304) is resistant for fairly longperiods in a high temperature oxidising environment: in particular themost common uses for this material are those which envisage stays in airup to about 500° C. The new steel was also tested for its resistance totemperatures higher than room temperature by means of air heating testsinside a muffle furnace. The results can be seen in FIG. 10.

The resistance was evaluated by measuring the depth of surfaceoxidation, i.e. the loss of diameter as a result of oxidation. It ispossible to note that the new steel behaves in a manner perfectlysimilar to that of the of the various types with a high nickel contentup to a temperature of higher than 800° C. As mentioned, thetemperatures commonly used for normal austenitic steels (belonging tothe family of 1.4301 steel) are about 500° C., while for highertemperatures refractory alloys (with high nickel contents) orsuperalloys (nickel based alloys, not belonging to the family of steels)are used. The new steel is therefore perfectly utilisable at the sametemperatures at which the basic type is used since there is no variationin its characteristics.

CONCLUSIONS

The new stainless steel according to the present invention with a lownickel content possesses technical characteristics similar or comparableto those of steel type 1.4301.

The main advantage of this new steel from the commercial point of viewis its lesser dependency on the nickel market and therefore its greaterstability from a price point of view. From the technical point of view,the main advantage is the extremely high suitability for drawing whichallows a large reduction during drawing and a small number ofintermediate annealing operations.

The new material is particularly suitable as a substitute fortraditional types of steel in certain specific applications

-   -   agricultural wire, owing to its optimum atmospheric corrosion        resistance and the excellent mechanical properties which can be        obtained;    -   glossy wire for domestic use, electric household appliances,        gratings, luggage racks, bicycle spokes, owing to the optimum        combination of corrosion resistance and mechanical strength in        the work-hardened state;    -   wire for laundry drying frames, owing to the good resistance to        saline mist (traces of chlorides may remain on the washed        laundry) and also good mechanical strength and non-magnetic        property;    -   special wires and screws for electronic components, owing to its        non-magnetic property in the deformed state and good cold        deformability;    -   wires for architecture, for meshwork and for hooks used on slate        roofs, owing to the mechanical strength and resistance to        environmental corrosion;    -   wire and tie-rods for industrial furnaces operating at a medium        to low temperature (up to 550° C., for treatment of copper,        aluminium and other alloys), owing to the excellent resistance        to temperatures up to 800° C.

1-18. (canceled)
 19. Austenitic stainless steel having the followingcomposition by weight: 0.03%<carbon<0.07% 7.0%<manganese<8.5%0.3%<silicon<0.7% sulphur≦0.030% phosphorus≦0.045% 16.5%<chromium<18.0%3.5%<nickel<4.5% 0.1%<molybdenum<0.5% 1.0%<copper<3.0%0.1%<nitrogen<0.3% the difference consisting in iron and impurities. 20.Austenitic stainless steel according to claim 19, wherein:0.04%<carbon<0.06%.
 21. Austenitic stainless steel according to claim19, wherein: 7.5%≦manganese<8.0%.
 22. Austenitic stainless steelaccording to claim 19, wherein: 0.4%<silicon0.6%.
 23. Austeniticstainless steel according to claim 19, wherein: sulphur<0.005%. 24.Austenitic stainless steel according to claim 19, wherein:0.002%<sulphur<0.004%.
 25. Austenitic stainless steel according to claim19, wherein: 0.030%<phosphorus<0.035%.
 26. Austenitic stainless steelaccording to claim 19, wherein: 17.0%≦chromium<17.5%.
 27. Austeniticstainless steel according to claim 19, wherein: 3.8%<nickel<4.2%. 28.Austenitic stainless steel according to claim 19, wherein: 4.0%<nickel.29. Austenitic stainless steel according to claim 19, wherein:0.1%<molybdenum<0.3%.
 30. Austenitic stainless steel according to claim19, wherein: 2.0%≦copper<2.5%.
 31. Austenitic stainless steel accordingto claim 19, wherein: 0.15%<nitrogen<0.2%.
 32. Austenitic stainlesssteel having the following composition by weight: 0.04%<carbon<0.06%7.5%≦manganese<8.0% 0.4%<silicon<0.6% 0.002%<sulphur <0.004%0.030%<phosphorus<0.035% 17.0%≦chromium<17.5% 3.8%<nickel<4.2%0.1%<molybdenum<0.3% 2.0%≦copper<2.5% 0.15%<nitrogen<0.2% the differenceconsisting in iron and impurities.
 33. Austenitic stainless steel havingthe following composition by weight: carbon about 0.055% manganese about7.50% silicon about 0.52% sulphur about 0.003% phosphorus about 0.032%chromium about 17.0% nickel about 4.0% molybdenum about 0.19% copperabout 2.0% nitrogen about 0.17% the difference consisting in iron andimpurities.
 34. Articles containing or consisting of austeniticstainless steel according to claim
 19. 35. Articles containing orconsisting of austenitic stainless steel according to claim
 32. 36.Articles containing or consisting of austenitic stainless steelaccording to claim
 33. 37. Articles according to claim 34 selected fromamong: wires for agricultural use, wires for domestic use, electrichousehold appliances, gratings, luggage racks, bicycle spokes; wires forlaundry drying frames; wires and screws for electronic components; wiresfor architecture, for meshwork and for hooks used on slate roofs; wiresand tie-rods for industrial furnaces.
 38. Articles according to claim 35selected from among: wires for agricultural use, wires for domestic use,electric household appliances, gratings, luggage racks, bicycle spokes;wires for laundry drying frames; wires and screws for electroniccomponents; wires for architecture, for meshwork and for hooks used onslate roofs; wires and tie-rods for industrial furnaces.
 39. Articlesaccording to claim 36 selected from among: wires for agricultural use,wires for domestic use, electric household appliances, gratings, luggageracks, bicycle spokes; wires for laundry drying frames; wires and screwsfor electronic components; wires for architecture, for meshwork and forhooks used on slate roofs; wires and tie-rods for industrial furnaces.40. A method of preparing an article selected from the group consistingof wires for agricultural use, wires for domestic use, electrichousehold appliances, gratings, luggage racks, bicycle spokes, wires forlaundry drying frames, wires and screws for electronic components, wiresfor architecture, for meshwork and for hooks used on slate roofs, andwires and tie-rods for industrial furnaces, said method comprisingpreparing said article using an austenitic stainless steel according toclaim
 19. 41. A method of preparing an article selected from the groupconsisting of wires for agricultural use, wires for domestic use,electric household appliances, gratings, luggage racks, bicycle spokes,wires for laundry drying frames, wires and screws for electroniccomponents, wires for architecture, for meshwork and for hooks used onslate roofs, and wires and tie-rods for industrial furnaces, said methodcomprising preparing said article using an austenitic stainless steelaccording to claim
 32. 42. A method of preparing an article selectedfrom the group consisting of wires for agricultural use, wires fordomestic use, electric household appliances, gratings, luggage racks,bicycle spokes, wires for laundry drying frames, wires and screws forelectronic components, wires for architecture, for meshwork and forhooks used on slate roofs, and wires and tie-rods for industrialfurnaces, said method comprising preparing said article using anaustenitic stainless steel according to claim 33.